Fat on Thighs and Paunches Is the Fate of All Mammals

By NATALIE ANGIER

Published: October 30, 1990

FEW things in life are as vexing as the talent that fat has for gathering in unsightly pockets and paunches at particular spots around the body: jiggling on the thighs, bulging out from the belly, flapping down from the triceps.

But distressing though such fat deposits are, they may simply be the price people pay for being mammals.

In a rash of new studies comparing human fat tissue with that of other mammals, researchers have found that, even in the leanest wild mammals, fat tends to be distributed in similar locations around the body.

From dissection of many species, researchers have discovered that fat distribution under the skin and around internal organs is far from continuous or uniform, as had long been believed. Instead, nearly all mammals, be they squirrels, badgers, deer, wolverines, camels or humans, tend to store fat in the same discrete hot spots.

The relative amounts of fat vary from one beast to another, but whatever the species, fat deposits collect in the breast area, around the upper part of the front legs (the upper arms of humans), on the tailbone and around the thighs, in three to eight regions of the abdomen and at the back of the neck.

In many mammals, a surprisingly sizable glob of fat surrounds the heart, a discovery that contradicts current notions that fat near the heart is a pathological condition largely confined to humans.

Researchers have also learned that the body's fat cells, or adipose tissue, is far more biochemically diverse than imagined. Adipose tissue displays notably different properties depending on its location in the body.

Some deposits are efficient at absorbing lipids, or fat molecules, from the bloodstream, while other deposits are primed to release lipids easily as fuel for neighboring tissue. The different fat deposits "may really be thought of as substantially different organs," said Dr. M. R. C. Greenwood, professor of nutrition and internal medicine at the University of California at Davis.

Scientists hope their cross-species studies of fat will yield new insights into human obesity, including a better understanding of why women are generally chubbier than men, and why people who gain pounds around the midriff are at a higher risk for cardiovascular disease than are those who gain in the thighs and buttocks.

Other researchers are comparing the enzymes used by various mammals to synthesize, process and store fat molecules. Of particular interest is an enzyme called lipoprotein lipase, which plays a lead role in extracting fatty acids after a meal and storing them in fat cells. Lipoprotein lipase has been detected in nearly every species examined, and it is more abundant in females than in males, probably to allow females to store fat easily for pregnancy.

The exquisite regulation of that and other enzymes may explain why bears, woodchucks and other animals can become exceedingly stout each year before hibernating or fasting without suffering the ill effects of obesity often seen in humans, like high blood pressure, clogged arteries and diabetes.

"A polar bear can eat so much seal blubber that the fat in its blood would instantly kill a dog or a rabbit," said Dr. G. Edgar Folk Jr., a professor of physiology at the University of Iowa in Iowa City. "But despite that high fat, it doesn't get clogged vessels or a fatty liver. It manages to use fat as an energy source without getting into trouble."

In the dreamiest of all possibilities, the new understanding could suggest better treatments for obesity. At the very least, scientists hope to convince people that all fat is not created equal, nor is all fat equally bad.

"The biological literature has been amazingly sparse about fat in wild animals," says Dr. Caroline M. Pond of the Open University in Milton Keynes, England. "But a number of wild animals are fat and proud of it, and we're finally getting the facts about how that fatness is controlled." The Nature of Fat A Storage System To Provide Energy

Whatever its biochemical peculiarities, the bottom line of fat for animals is to serve as a handy store of energy in hard times. Fat in the diet is almost effortlessly converted into fat on the body. Fat molecules provide the fuel creatures need to function, and because their chemistry allows them to merge and break apart easily, they are the best form of stored fuel. When the digestive system converts consumed fat into storable fat, it spends only 2 percent of the fat molecules' inherent energy in the process, allowing the rest to be dropped off in fat cells.

What is more, the adipose tissue that stores fat molecules for future use can expand almost indefinitely, an unusual property shared by no other organ save the skin. Part of that flexibility results from the nature of the fat cells that make up adipose tissue. Individual fat cells can balloon up to 10 times or more their original size. Fat Cells Are Forever

"A fat cell is just one big round fat droplet inside a membrane," said Dr. James O. Hill, a pediatrician and an expert in lipid metabolism at Vanderbilt University School of Medicine in Nashville, Tenn. "It can get as big or as little as it wants."